错误：未捕获（承诺）：TypeError：在实现离子网络功能时，Object（...）不是函数

Question

当我尝试实现以下功能时，我遇到了这个问题。任何人都可以帮助我解决此错误

     this.network.onDisconnect().subscribe(() => {
          console.log('network was disconnected :-(');
         });

         this.network.onConnect().subscribe(() => {
          console.log('network was connected :-(');

          });